The present invention relates generally to mobile radiocommunications systems.
The present invention relates more particularly to apparatus for measuring the distance, or the propagation time, between a mobile station and a base station in such a system.
Such knowledge of distance or of propagation time can be used for various purposes, such as the following, given by way of example:
in a mobile radiocommunications system of the Time Division Multiple Access (TDMA) type, such as in particular the Global System for Mobile communications (GSM), such knowledge can be used for the purpose of determining the timing advance to be applied to information from the mobile station so as to enable said information to be received at the base station in that one of the time channels which has been allocated to said mobile station, regardless of the propagation time between said mobile station and said base station; and
in a mobile radiocommunications system of the cellular type (also such as the above-mentioned GSM), such knowledge can be used for the purpose of controlling the transmission power of the mobile station as a function of the distance between it and the base station so as to reduce the overall interference level in the system, or else so as to locate the mobile station, e.g. by combining the result of such a measurement of the distance between the mobile station and a base station with the results of measurements of the distances between said mobile station and other base stations.
To determine the propagation time, or the distance, between a mobile station and a base station in a mobile radiocommunications system, it is known to be possible to determine the reception instant at which predetermined data transmitted by the base station is received by the mobile station, such predetermined data being, in particular, a xe2x80x9ctraining sequencexe2x80x9d (used in known manner to estimate the transmission channel prior to equalizing the signals received over said transmission channel). Once said reception instant has been determined, the base station can be informed of it, and, by comparing it with the transmission instant at which said data was transmitted, and also given that the mobile station is synchronized continuously on the base station, said base station can deduce the propagation time, i.e. the looked-for distance.
To determine such a reception instant at which the mobile station receives predetermined data, it is also known to be possible to correlate the modulated signal as received by the mobile station with a reference signal generated in the mobile station and corresponding to said predetermined data. In known manner, such correlation is performed on signals that have been sampled and digitized, and it involves performing similarity measurements on the two signals for various positions in time of one signal relative to the other, the positions being obtained by shifting the reference signal relative to the received signal by one sampling period each time. That one of the positions for which the similarity between the two signals is the greatest is thus representative of the reception instant at which the mobile station receives said predetermined data.
Using known methods, the accuracy of an instantaneous measurement of the propagation time cannot be better than the sampling period (or the accuracy of an instantaneous measurement of distance cannot be better than the product of the sampling period multiplied by the propagation speed of the signals). By way of example, for a mobile radiocommunications system such as the above-mentioned GSM, the accuracy of such a distance measurement is thus about one kilometer.
An object of the present invention is to improve that accuracy.
The present invention provides apparatus for measuring the distance between a mobile station and a base station in a mobile radiocommunications system, said apparatus including means for determining the reception instant at which the mobile station receives predetermined data transmitted by the base station, which means themselves include correlation means for correlating an in-phase component and a quadrature component of a modulated signal received by the mobile station respectively with an in-phase component and with a quadrature component of a reference signal generated in the mobile station and corresponding to said predetermined data, said correlation means delivering real components and imaginary components of correlation coefficients, and said apparatus being characterized in that it further includes means for using the real components and the imaginary components of the correlation coefficients to determine a complex magnitude whose phase varies continuously as a function of said reception instant.
The present invention further provides a mobile station including such means for measuring the reception instant at which the mobile station receives predetermined data transmitted by a base station in a mobile radiocommunications system.